A. Field of the Invention
This invention relates generally to whiteners, and more particularly to improved whitener pigment materials for use in papermaking and other applications, as well as methods and apparatus for making these materials.
B. Description of the Related Art
Whitener pigments are employed in a diverse variety of industrial, business and decorative environments to produce a "white" visual impression. Such pigments may be mixed with suitable carrier materials to create, for example, white paper, plastic or paint. Whiteners produce their characteristic visual appearance by reflecting and scattering most visible frequencies of incident daylight radiation.
One popular whitener pigment is titanium dioxide, commonly referred to as "titania", TiO.sub.2. Crystals of this compound, particularly those having a rutile crystal structure, exhibit a high index of refraction and are largely inert chemically.
The performance of a particular pigment, as measured by the degree of apparent whiteness conferred by a given particle concentration, is often limited by the particles' physical characteristics. For example, irregularities in particle shape and size can reduce the opacity (or covering power) of the resulting whitener composite. For a given refractive index, there is an optimum size to achieve the maximum opacity. In the case of spherical particles of rutile titania, the optimum size is 0.2 to 0.3 .mu.m in diameter.
Unfortunately, it is difficult to control the morphologies of typical whitener-pigment crystals. Conventional production methods generate particles having different shapes and a range of sizes, with concomitantly limited performance attributes.